RFR: 2086: Use short Ids to get cached issue data in CheckWorkItem#getIssueMetadata

[Zhao Song]

A user reported that in a repo which is configured with maintainer approval feature, he added the approved label to the issue but skara bot didn't update the related PR.

>From the log, I found skara bot said "[Issue]No activity since last check, not checking again."

When investigating, I guess the scenario likes this. When the bot is evaluating a PR. The bot fetches the issue and the issue only contains label “XXX-fix-request”. Then, the user add "XXX-fix-yes" to issue. So in the end of evaluation, the bot fetches the issue again and calculates the metadata again (at that time, the issue contains label “XXX-fix-request, XXX-fix-yes”). So in the next round, the bot would think that new label “XXX-fix-yes” is already handled, so it will not evaluate the PR again.

But as Erik introduced a cache for IssueTrackerIssue in CheckWorkItem in [SKARA-1963](https://bugs.openjdk.org/browse/SKARA-1963). I think the bot shouldn't fetch the issue from JBS again in the end of checkRun. But the bot really did. Then I found that the key of the map(cache) are supposed to be short id of the JBS issue, But in CheckWorkItem#getIssueMetadata, the bot is trying to use the whole id of JBS issue to get the cached data, which will always miss and trigger the remote call.

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Commit messages:
 - SKARA-2086

Changes: https://git.openjdk.org/skara/pull/1579/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1579&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2086
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1579.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1579/head:pull/1579

PR: https://git.openjdk.org/skara/pull/1579